I really like my Custom 50. How did the tone change with the EL34s? Did they reduce the gain?
11top, forgive me if I'm telling you what you already know, I'm just throwing this out there as general info:
EL34s
can increase the perceived tightness of an amp, and give a more focused crunch, because EL34s tend to break up more in the midrange and have a bit less bass.
It's not that they necessarily reduce or increase the distortion, it's that they put it in a different part of the frequency spectrum. And different EL34s and 6L6s will break up at more or less volume depending on the tube.
So you could get higher gain with a particular 6L6 or a particular EL34, but things will vary from tube to tube.
But before swapping power tubes this is also true: the more you depend on preamp gain, the less a power tube swap will make a noticeable difference. If you're running your preamp to produce high gain as is done with most modern two channel amps, it's all about the preamp tubes, and you'll honestly get more bang for your buck experimenting on the front end of the amp.
Example: I had a Mesa Tremoverb that ran either tube, and did quite a bit of experimentation swapping them, and ultimately I concluded that it didn't matter which tube I used, the differences were about as audible as if I'd simply tweaked a tone control. But of course the Mesa was all about the preamp section of the amp, as its nearly 100 watts cranked was deafening.
I've had other amps where the power tubes really did make a difference, but they were different kinds of beasts designed to use power tube distortion.
However, if you run less preamp gain, and prefer to crank the amp for power tube distortion, you'll hear differences between power tubes. But an increase or decrease in gain will depend on the design and production of the tube itself, and not necessarily the designation of EL34 or 6L6. As an example, NOS Philips 6L6 WGBs will break up faster than an EL34, and can sound very similar, whereas most 6L6s have more bass, and more clean headroom, than most EL34s.
As an example, you see clean amps like Twins with 6L6 glass, contrasted with gainier amps like Marshalls using EL34s. But a lot also depends on the preamp section and tone stack. Then you have amps like Vox AC30s that use those little glass EL84s to break up the amp, and that gets very loud, but you can't simulate that "crushed glass" thing with preamp tube breakup IMHO (however the EF86 tube in the preamp section seems to matter).
Again, forgive me if I'm preaching to the choir, and you already know this.
